1. Field of the Invention
The present invention relates in general to differential input stages, and more particularly to a precision, low power, high voltage differential input stage, with incorporated protection against DC and fast AC transient signals at the input.
2. Description of the Related Art
It is desired to provide a precision, high voltage, low power input stage for analog amplifiers and comparators. Also, it is desired to provide gate protection for the transistors for large direct-current (DC) voltage differences, sudden large signal alternating-current (AC) transients, and protection against electrostatic discharge (ESD) pulses for both powered and unpowered circuits.
It is possible to fabricate high voltage differential input stages for amplifiers or comparators using lateral bipolar PNP transistors. Lateral bipolar PNP transistors do not need input protection because of their large base-emitter breakdown voltage. Lateral bipolar PNP transistors, however, are relatively slow, and separate ESD circuits are required to protect the inputs. Furthermore, the base current of a lateral bipolar PNP transistor shows up as input bias current, so that it is generally not suitable for low power applications. Moreover, high voltage lateral bipolar transistors are not commonly available in modern integrated circuit (IC) process manufacturing technologies.
Amplifier configurations using high voltage MOSFETs can be fast and have negligible input bias currents. But such configurations lack the DC precision that is achievable with bipolar designs. This is the result of the inherent poor matching of high voltage MOSFET transistors. Matching is the degree to which two devices behave exactly the same under identical operating conditions. Also, amplifier circuits built with high-voltage MOSFETs conventionally require gate protection circuits that further decrease the accuracy of the input stages.